An ion-conductive electrolyte membrane (hereinafter, sometimes referred to simply as “electrolyte membrane”) is used in a fuel cell, for example, and an oxygen ion-conducive electrolyte membrane is used in a membrane electrode assembly of a solid oxide fuel cell, for example. Such membrane electrode assembly is constructed by joining a fuel electrode (hydrogen electrode) to one side of a solid oxide electrolyte membrane which is an electrolyte membrane, and an air electrode (oxygen electrode) to the other side. In the solid oxide fuel cell having such membrane electrode assembly, hydrogen or carbon monoxide is supplied to the fuel electrode while oxygen or air is supplied to the air electrode. With the membrane electrode assembly heated, oxygen gains electrons at the air electrode to form oxygen ions. The oxygen ions permeate through the electrolyte membrane to reach the fuel electrode, and react with hydrogen (or carbon monoxide) to form water (water vapor) or carbon dioxide and release electrons. The electrons travel through a load, thus supply electric power to the load, and when the electrons reach the air electrode, the electrons ionize oxygen supplied to the air electrode.
In such solid oxide fuel cell, if the electrolyte membrane constituting part of the membrane electrode assembly has a defect, such as a pin hole or a crack, gas leaks through the electrolyte membrane, resulting in a reduction in electric power generation capacity. If, for example hydrogen gas is supplied to a space facing one side of the electrolyte membrane, such defect allows the hydrogen gas to leak through it to the other side of the electrolyte membrane. Thus, the presence of a defect can be detected by measuring the concentration of leaked hydrogen gas with a hydrogen sensor. The hydrogen sensor for use in such measurement can be formed using a hydrogen storing alloy, for example, as disclosed in Unexamined Japanese Patent Publication No. 2004-233097, for example.
In a solid oxide fuel cell having a plurality of membrane electrode assemblies electrically connected in series to increase output voltage, maximum output current is determined by the membrane electrode assembly lowest in oxygen ion conductivity. It is therefore desirable that the membrane electrode assemblies connected in series be as uniform in oxygen ion conductivity as possible. Thus, a technique of estimating the oxygen ion conductivity of an electrolyte membrane by attaching a metal electrode to either side of the electrolyte membrane and measuring an electrical characteristic of the electrolyte membrane, such as AC impedance, has been developed, as proposed in Unexamined Japanese Patent Publication No. 2006-286397, for example.
In the examination method by detecting leaked hydrogen gas diffused in an atmosphere, an ability for detecting a defect of the electrolyte membrane lowers due to the diffusion of leaked hydrogen gas. In addition, detection of leaked hydrogen gas in an atmosphere does not give the location of the defect.
The measurement of an electrical characteristic (AC impedance or the like) of an electrolyte membrane is not direct measurement of oxygen ion conductivity; it only gives an estimate of oxygen ion conductivity. In addition, whether or not the electrolyte membrane has uniform oxygen ion conductivity in every region cannot be examined by this measurement.